The present invention concerns monitoring devices for films on substrate surfaces and, in particular, a device for optically determining the characteristics of a thin film on the surface of a glass sheet.
Films are typically applied to glass to either absorb solar radiation or to reflect solar radiation. There are many applications both in industry and in research for an instrument to measure the thickness of thin films. Various devices are available commercially for this purpose. For example, surface profile monitors measure the step size, at a point where the film has been etched through to the substrate, by tracking a stylus over the surface. Physical contact can distort or damage the surface so that a non-contact measurement method is in many cases desired. Methods based on the reflection of light from a film surface offer such a non-contact probe.
There are essentially two distinct techniques for the optical determination of film thickness. The first, known as ellipsometry, measures and compares the reflection coefficient at a given wavelength and an angle to incidence for polarization perpendicular and parallel to the plane of incidence. Considerable computation is required and unambiguous results are obtained only if it is known a priori that the film thickness lies within a restricted range, usually 0 to about 3,000 angstroms. The second class of optical techniques is based on the wavelength and/or angle of incidence dependence of the reflectivity, observable as the well-known interference colors in thin films. Although the principle of this technique is simple, instruments based on this principle are not inexpensive and the simplest instruments do not even provide a direct reading of the film thickness.
An optical thickness gauge that measures the thickness of transparent or semi-transparent sheet material, by the reflectivity technique used in the second class of prior art techniques described above, is disclosed in U.S. Pat. No. 2,655,073. This gauge reflects light from a specimen sheet material onto a rotatable flat optical reflector having parallel surfaces to form interference fringes when the reflected light beams are in phase. The flat optical reflector is rotated in the path of the reflected beam from the specimen beam to the angle at which the light beams are in phase to form the fringes for display on a mirror. The angle of rotation of the reflector is the measure of the thickness of the sheet material. This patent also describes the use of an optical wedge which is utilized to form interference fringes when the reflected beams are in phase wherein the thickness of the optical wedge coincides with the thickness of the specimen.